The following questions and answers were created from interviews with experts on radiation poisoning, nuclear-reactor design and past nuclear accidents:

Q: What sorts of radioactive materials are we talking about, and how harmful are they?

A: In the current situation, radiation can come from at least four sources: the uranium fuel that is placed inside the fuel rods; the spent uranium rods that are now exposed in Reactor No. 4; the dispersal or venting of gases built up inside the reactors from a chemical reaction that takes place between the rods and their alloy casing if the rods become too hot; and the seawater that is being used to cool the reactors.

Q: What is the potential for release from the spent fuel rods, which are currently uncovered and no longer submerged in water?

A: The spent fuel rods are kept under water to stop them from burning through their casings, which are made of zirconium. In at least one of the spent fuel ponds, the casings have began to burn, exposing the fuel rods and releasing cesium into the air. Since there are no explosions, or ventilation, the cesium will likely form a layer around the area of the pond and contaminate it.

Q: How do they stop that?

A: The only way to stop that process is to resubmerge the fuel rods — this can be done by helicopters or by robotic means. If the cesium buildup continues, workers trying to get close to the reactors will be exposed to lethal doses of radiation.

The water will then have to be mixed with boric acid to stop the burnt uranium from mixing with water and reaching what scientists call “re-criticality.” At that point, the fuel rods themselves become a sort of open-air reactor, releasing radiation into the sky.

Q: How much of that has been released, and what are the effects of that?

A: The uranium fuel rods inside each reactor are still expected to be largely intact. If the reactors are cooled down over time, the rods will be disposed of as radioactive waste. The spent rods, which were usually stored in a pool, are radioactive — or hot — and will be leaking radiation until they are fully submerged with cool, distilled water and return to a normal temperature.

At least one of the pools of spent fuel rods is completely dry and out of control.

Trace amounts of cesium-137 and iodine-131 were found in the air around the plant starting Saturday, around 1:30 p.m. local time. The two isotopes are produced when the fuel rods inside the reactors overheat and react with their casings. Both are radioactive and can cause health damage.

Iodine has a half-life, or reduces in mass — and thus, radioactivity — of eight days. Within two months, it leaves no trace behind. It can be countered with doses of potassium-iodide tablets, but only within 24 hours of exposure. It is known to cause thyroid cancer.

Cesium has a longer half-life, of about 300 years. External exposure to large amounts of Cs-137 can cause burns, acute radiation sickness and even death. Exposure to Cs-137 can increase the risk of cancer because of exposure to high-energy gamma radiation. Internal exposure to Cs-137, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, exposing these tissues to the beta particles and gamma radiation, and increasing cancer risk.

Q: How much of this material is out there?

A: So far, very small amounts of cesium and iodine are reported to have been released, mostly in a process when engineers are venting pressurized gases out from near the reactor core to chambers outside it, where leaks and explosions have spread it out.

How much cesium or iodine remains inside the nuclear reactors is the big unknown — the fuel rods have been partially submerged several times, and the temperatures have risen each time. If engineers can get control of the reactors, and finish the cooling process, then this radioactive material should be entombed within the reactor itself.

If the cooling process fails for any reason, and the containment chambers are sufficiently damaged, this material could leak out. This is the worst-case scenario.

Sources: World Nuclear Association, the Science Media Centers of Japan and Australia, the World Health Organization in Geneva, the US Centers for Disease Control and Prevention, the Environmental Protection Agency